feat(#37): Enable tracking credits usage with proof of concept test page

supabase/migrations/20240929120431_add_credits.sql

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+create table user_credits (
+  user_id uuid references auth.users not null primary key,
+  credits_remaining integer not null default 0,
+  last_updated timestamp with time zone not null default now()
+);
+
+alter table user_credits enable row level security;
+
+create policy "Can view own credits." on user_credits
+  for select using (auth.uid() = user_id);
+
+-- Uncomment and modify the update policy if allowing users to modify their own credits
+-- create policy "Can update own credits." on user_credits
+--   for update using (auth.uid() = user_id);
+
+create table credit_transactions (
+  id uuid default uuid_generate_v4() primary key,
+  user_id uuid references auth.users not null,
+  credits_change integer not null,
+  description text,
+  created_at timestamp with time zone not null default now()
+);
+
+alter table credit_transactions enable row level security;
+
+create policy "Can view own transactions." on credit_transactions
+  for select using (auth.uid() = user_id);
+
+-- Adding indexing to improve query performance
+create index on user_credits (user_id);
+create index on credit_transactions (user_id);
+
+-- Adding a check constraint to enforce data integrity
+alter table credit_transactions
+add constraint check_credits_change_nonzero check (credits_change <> 0);